Liquid crystals currently are used in a wide variety of devices, including optical devices such as visual displays. Such devices usually require relatively low power and have a satisfactory response time, provide reasonable contrast, and are relatively economical. The property of liquid crystals enabling use, for example, in visual displays, is the ability of liquid crystals to transmit light on one hand, and to scatter and/or absorb light, on the other, depending on the alignment (or lack of alignment) of the liquid crystal structure with, e.g., an electric field applied across the liquid crystal material. An example of electrically responsive liquid crystal material and use thereof is provided in U.S. Pat. No. 3,322,485.
Certain liquid crystal material is responsive to temperature, changing optical characteristics in response to temperature of the liquid crystal material.
The present invention is disclosed hereinafter particularly with reference to the use of liquid crystal material that is particularly responsive to an electric field as well as to temperature.
Currently there are three categories of liquid crystal materials, namely cholesteric, nematic and smectic types. The invention of the present application relates in the preferred embodiment described below to use of liquid crystal material having a smectic phase. The invention may also be employed with a liquid crystal material having a smectic as well as a nematic phase.
The various characteristics of the cholesteric, nematic and smectic types of liquid crystal material are described in the prior art. One know characteristic of liquid crystal material is that of reversibility; in particular it is noted here that nematic liquid crystal material is known to be reversible, but cholesteric material is not reversible. One characteristic of a reversible nematic material is that the liquid crystal structure will return to its original configuration after an electric field, for example, has been applied and then removed. On the other hand, smectic material will retain its configuration even after removal of an electric field.
To enhance contrast and possibly other properties of liquid crystal material, pleochroic dyes have been mixed with the liquid crystal material to form a solution therewith. The molecules of the pleochroic dye generally align with the molecules of the liquid crystal material. Therefore, such pleochroic dyes will tend to function optically in a manner similar to that of the liquid crystal material in response to a changing parameter, such as application or non-application of an electric field or heat. Examples of the use of pleochroic dyes with liquid crystal material are described in U.S. Pat. Nos. 3,499,702 and 3,551,026.
An important characteristic of liquid crystal material is anisotropy. An anisotropic material has different physical properties in different directions. For example, liquid crystals are optically anisotropic, i.e., they have indices of refraction which vary with the direction of propagation and polarization of the incident light.
Liquid crystal material also has electrical anisotropy. For example, the dielectric constant for smectic liquid crystal material may be one value when the molecules in the liquid crystal structure are parallel to the electric field and may have a different value when the molecules in the liquid crystal structure are aligned perpendicular to an electric field. Since such dielectric value is a function of alignment, for example, reference to the same as a "dielectric coefficient" may be more apt than the usual "dielectric constant" label. Similar properties are true for other types of liquid crystals.
Some brief discussion of the encapsulation of cholesteric liquid crystal material is presented in U.S. Pat. Nos. 3,720,623, 3,341,466, and 2,800,457, the latter two patents being referred to in the first named patent.
In the past, devices using liquid crystals, such as visual display devices or other devices, have been of relatively small size. Large size devices using liquid crystals, such as, for example, a billboard display or a sign have not been satisfactorily fabricatable for a number of reasons. One reason is the fluidity of the liquid crystals, (the liquid crystal material may tend to flow creating areas of the display that have different thicknesses). As a result, the optical characteristics of the display may lack uniformity, have varying contrast characteristics at different portions of the display, etc. The thickness variations in turn cause variations or gradations in optical properties of the liquid crystal device. Moreover, the varying thickness of the liquid crystal layer will cause corresponding variations in the electrical properties of the liquid crystal layer, such as capacitance and impendance, further reducing uniformity of a large size liquid crystal device. The varying electrical properties of the liquid crystal layer, then, also may cause a corresponding variation in the effective electric field applied across the liquid crystal material and/or in response to a constant electric field would respond differently at areas of the liquid crystal that are of different thicknesses.
A pleochroic display, i.e., one in which pleochroic dye and liquid crystal material are in solution together, has the advantage of not requiring the use of a polarizer. However, such a pleochroic device has a disadvantage of relatively low contrast when only nematic liquid crystal material is used. It was discovered in the past, though, that a cholesteric liquid crystal material could be added to the nematic one together with the dye to improve the contrast ratio. See White et al. article, Journal of Applied Physics, Volume 45, No. 11, November 1974, at pages 4718-4723, for example. The cholesteric material would tend not to return to its original zero field form when the electric field is removed.
Another problem encountered with pleochroic dye included in solution with liquid crystal material, regardless of the particular type of liquid crystal material, is that the light absorption of the dye is not zero in the "field-on" condition. Rather such absorption in the "field-on" condition follows a so-called ordering parameter, which relates to or is a function of the relative alignment of the dyes. The optical transmission characteristic of liquid crystal material is an exponential function of the thickness of the liquid crystal material. Specifically, the "on" state or "field-on" or "energized" state of the liquid crystal material is an exponential function of the thickness of the liquid crystal material, and the "absorbing" state or "field-off" state also is a different exponential function of the thickness.
To overcome those problems described in the two immediately preceding paragraphs, the liquid crystal material should have an optimum uniform thickness. (As used herein the term "liquid crystal" material means the liquid crystals themselves and, depending on context, the pleochroic dye in solution therewith). There also should be an optimum spacing of the electrodes by which the electric field or heat is applied to the liquid crystal material. To maintain such optimum thickness and spacing, rather close tolerances must be maintained. To maintain close tolerances, there is a limit as to the size of the device using such liquid crystals, for it is quite difficult to maintain close tolerances over large surface areas, for example.